ON THE DESTRUCTION OF ROCKS. 



Of the Desquamation of Granite. 



In some columns of red granite brought from 

 Leptis in Africa and lately in the British museum, 

 the shafts are in the act of desquamation ; casting off 

 crusts similar to those which are occasionally seen in 

 natural blocks of granite, and equally resembling, 

 except in their superior, integrity, those which are 

 found on the surfaces of the columnar trap rocks after 

 exposure to the weather. It is obvious that the form 

 of the shafts can bear no relation to the original forms 

 of the blocks of granite from which they had been 

 wrought; and it necessarily follows, that this desqua- 

 mation can not depend on an internal concretionary 

 structure, but must have resulted from the action of 

 the weather on the exposed surfaces. 



It is worthy of remark, that, in this case, the de- 

 tached crust is not decomposed, and that, except in 

 tenderness and fragility, it appears scarcely changed 

 from its natural state, Nor is any stratum of clay or 

 decomposed matter found at the place where the crust 

 separates from the solid block; at least none such 

 was found in the places which I had an opportunity 

 of examining. 



The decomposition of rocks has in most cases, and 

 with justice, been attributed to changes in the state 

 of the iron entering into them ; but it is evident, 

 that neither this, nor the other causes which have 

 been supposed to produce that effect, are capable of 

 explaining the very singular process in question ; as 

 it is not conceivable how the ordinary action of the 

 atmosphere should affect the interior part of the stone, 

 while the surface more immediately exposed to its 

 agency has escaped. Now, in examining with atten- 



